b'Carnegie Science|Summer 2019 11Edwin Hubbles LegacyIn 1914, Edwin Hubble, the most famous astronomer of the 20thThis plate marked VAR! was taken by Edwin century, decided to pursue his passion for astronomy and beganHubble on the 100-inch Hooker telescope in October 1923. Red VAR! marks the first his Ph.D. at the University of Chicago. In 1917, during the lastCepheid variable star found in Andromeda, stages of his degree, George Ellery Hale, founder of then-Carnegieswhich led to the discovery that there are Mount Wilson Observatory, asked him to join the staff. But Worldgalaxies outside of the Milky Way. The discovery made the universe vastly bigger War l called and Hubble enlisted in the U.S. Army instead. than known before. After the war, Hubble did join Mount Wilson in 1919, the sameImage courtesy the Carnegie Observatoriesyear that Andrew Carnegie died. Hubble began observing on the most powerful telescope on Earth, the 100-inch Hooker. The rest is history, as they say: Hubble began to systematically transform our understanding of the cosmos. In the fall of 1923, Hubble saw what he believed to be an exploding nova star in the Andromeda galaxy. At that time, the known universe was thought to be just the Milky Way. After comparing the bright object to older photographic plates, it became clear to Hubble that it was a Cepheid variable star. These objects have regular cycles of brightness; the frequency is related to its luminosity, making them valuable for calculating cosmic distances. The calculations showed that the object was far outside the Milky Way. Suddenly, the discovery confirmed that the universe was vastly bigger. In 1929, Hubble made another cosmos-shattering discovery. His measurements showed that all galaxies were moving away from us and their velocities increased with their distance. Clearly, the universe is expanding, a discovery dubbed Hubbles law.Hubbles legacy has continued at Carnegie ever since. In the 1930s, Walter Baade and Rudolph Minkowski showed that certainHubble Constant in 2001 using the Hubble Space Telescope to supernovae, brilliant stellar explosions that persist for weeks, werereach more distant Cepheid variables.similar in duration and variation. They also had similar spectra Today this legacy persists with the Carnegie Supernova wavelengths revealing chemistry and age.Project (CSP). Using near-infrared and visual observations, the In 1939, Carnegies Olin Wilson suggested that superbrightgoal of the CSP is to improve the precision of Type la supernova supernovae from two-star systems, now called Type la supernova,distances to better than 5%. Many of the most fundamental could be used to study the expanding universe. In the 1980s, digitaldiscoveries from this project have depended on the telescopes detectors made this practical.and instruments at Carnegies Las Campanas Observatory. Carnegies Allan Sandage encouraged astronomers, includingCarnegies Chris Burns heads a CSP effort to publish the Mark Phillips, to study the expansion of the universe using Type lamost precise Hubble diagrams, depicting velocity and distance. in the early 1990s. Phillips devised a method to measure distancesObservations also reveal how the explosions occur. Tony Piro to these objects with 10% or better precision. He was also a membermodels how the brightness increases at the onset of explosion to of one of the two teams who, in 1998, found that the expansion oftrack the distribution of radioactive nickel in the ejecta. The the universe is accelerating. That discovery pointed to a repulsiveteam hopes to provide an independent estimate of the dark force called dark energy.energy content of the universe, inspirational work that will Carnegie astronomer Wendy Freedman and team refined thecontinue to define the field for many years to come.continued on page 12Carnegie Supernova ProjectToday, the Carnegie Supernova Project members carry on Hubbles legacy by trying to understand the chemical evolution of the universe. Type Ia supernovae are essential to this work. They are the most powerful observational tools to measure the expansion of the universe and to probe the nature of dark energy. Mark Phillips is in the back row, center. Chris Burns is in the back row third from right, while Tony Piro is next to him, fourth from right. One Type la supernova is shown at the white bars in the spiral galaxy at right. Images courtesy Carnegie Supernova Project'